High Speed Uplink Packet Access (HSUPA) technique is introduced in 3GPP Release 7 for 1.28 Mcps option (i.e. TD-SCDMA system). This technique is mainly characterized by the introduction of Enhanced Dedicated Channel (E-DCH) for the uplink as well as several related PHY channels, and the use of Fast Node-B scheduling, Hybrid Automatic Repetition Request (HARQ), etc., so as to dramatically increase the capacity of the uplink.
Depending on whether a pre-assigned mode or an on-the-fly scheduling mode is employed by a User Equipment (UE) in the transmission of the uplink traffics, HSUPA may be divided into two modes: an on-schedule mode as a first mode, wherein UE transmits on a corresponding PHY channel according to an indication on the E-AGCH channel by a Node B (Base Station) whenever it transmits uplink data on E-DCH channel; and a non-schedule mode, wherein UE does not need to listen on E-AGCH channel, since the Node B has assigned a fixed PHY channel to the UE when assigning the E-DCH channel thereto.
In TD-SCDMA system, HSUPA relates to downlink PHY channels comprising E-DCH Absolute Grant Channel (E-AGCH) and E-DCH HARQ Indicator Channel (E-HICH); and relates to uplink PHY channels comprising E-DCH Physical Uplink Channel (E-PUCH), E-DCH Uplink Control Channel (E-UCCH), and E-DCH Random Access Uplink Control Channel (E-RUCCH). E-AGCH is for Node B to transmit schedule signaling, which comprises UE identification, PHY channel parameters, etc. For the on-schedule mode, said E-AGCH is also for the transmission of power control and sync control commands for E-PCH channel. E-HICH is for the transmission of feedback information for E-DCH channel. For a non-schedule user, said E-HICH is also for the transmission of uplink power control (TCP) and uplink sync control (SS) commands, etc. E-UCCH is for the transmission of signaling relating to uplink E-DCH transmission. E-PUCH is for the transmission of data of E-DCH and E-UCCH. E-RUCCH is for the UE to request for physical resources from the network side.
As shown in FIG. 1, for the on-schedule mode, HSUPA process comprises the following steps:
1. transmitting, by Node B, the identification of a UE and the related PHY channel parameters on E-AGCH;
2. transmitting, by the UE, upon hearing physical resource allocation thereto on E-AGCH, E-DCH data and the related uplink control information (E-UCCH) on the corresponding physical resource after appropriate delay (the specific delay is specified by the protocol);
3. transmitting feedback information by the Node B using a corresponding signature sequence after receiving the E-DCH data and appropriate delay according to whether the reception is correct or not, the feedback information comprising an Acknowledgement (ACK) or a Negative Acknowledgement (NAK), and an ACK is transmitted when Node B receiving the E-DCH data correctly, otherwise a NAK is transmitted.
The non-schedule mode is different from the on-schedule mode. And the differences mainly lie in that: in non-schedule mode, the PHY channel which is used by the UE to transmit E-DCH data is pre-assigned by the Node B, thus these is no need to listen on the E-AGCH channel; and that in addition to the feedback of ACK/NAK information, the Node B also needs to feedback power control and sync control commands on E-HICH.
In the HSUPA process above, the feedback information of the Node B for multiple UEs are transmitted on E-HICH. The feedback information for different UEs are spread using different “signature sequence”. The choice of a signature sequence is in exact correspondence to the PHY channel (E-PUCH) parameter assigned to the UE for transmission of E-DCH data, therefore the respective UEs may know the signature sequence for itself according to the PHY channel assigned thereto, so that the feedback information for itself may be detected on E-HICH.
A signature sequence is obtained from a 80×80 orthogonal matrix C80, the kth row of which is the kth signature sequence. Therefore, each signature sequence is of 80-bit long, and the sequential number of a signature sequence equals to its corresponding row number in the C80 matrix. C80 is formed by the tensor product, or Kronecker product, of two Hadamard matrixes, i.e. C80=C20{circle around (x)}C4, wherein {circle around (x)} denotes a tensor product, and wherein C20 is a 20×20 Hadamard matrix, and C4 is a 4×4 Hadamard matrix.
For the on-schedule mode, Node B transmits on E-HICH channel the feedback information ACK/NAK for the E-DCH data transmitted by the UE. The feedback information after coding is of 1 bit. The feedback information of the Node B for a UE is spread by the corresponding signature sequence for the UE, QPSK modulated, and further spread by a spreading code, and added with the feedback information for the other UEs before transmission, wherein C80,r denotes the rth signature sequence, and r is determined as follow:
      r    =                  16        ⁢                  (                                    t              0                        -            1                    )                    +                        (                                    q              0                        -            1                    )                ⁢                  16                      Q            0                                ,wherein t0 is of the first (lowest) timeslot assigned to the UE for the transmission of E-DCH data, t0=1, 2, . . . , 5; wherein Q0 is the spreading factor assigned to the UE for the transmission of E-DCH data in timeslot t0, Q0=1, 2, 4, 8, 16; and wherein q0 is the allocated chip, q0=1, 2, . . . , Q0.
For the non-schedule mode, not only the feedback ACK/NAK for the E-DCH data transmitted by the UE is transmitted on E-HICH by the Node B, but also the power control (TPC) and sync control information (SS) for E-DCH channel is feedback. Here, the 80 signature sequences are divided into 20 groups, each group comprising 4 signature sequences. The first signature sequence in each group is used for the spreading of the feedback information ACK/NAK, and the remaining 3 signature sequences and the respective complement code thereof, i.e. 6 codes in total, form the 6 states representative of TPC/SS, each state identified by one bit. After being spread by the Node B using a corresponding signature sequence, the feedback information and TPC/SS commands for UE are QPSK modulated, spread by a spreading code, and added with the feedback information for the other UEs before transmission. For the non-schedule mode, the assignment of signature sequence to an UE is signaled by a higher level, rather than calculated by a fixed equation.
As can be seen from the method for assigning E-HICH channel signature sequence, for the on-schedule mode, when the PHY channel assigned to a UE for the transmission of E-DCH data is fixed, the signature sequence is fixed; while for the HSUPA process in the non-schedule mode, a higher level signals the signature sequence assigned to the UE, and the signature sequence remains unchanged during this HSUPA process.
The analysis on the cross-correlation of the signature sequences shows that, when two signature sequences are cross-correlated with a shift of one bit therebetween, the resulting cross-correlation is related to the numbering of the two sequences. When the difference between the numbering equals to or is less than 8, the cross-correlation between the two sequences is relatively large; and when the difference is larger than 8, the cross-correlation is relatively small. Such characteristic is related to how the signature sequences are formed. As known to those skilled in the art, the larger the cross-correlation is between two signals, the more difficult it is to detect them. This is due to the fact that a wireless channel is typically a multi-path channel, which means that the signal received at the receiving end is the accumulation of versions of the signal transmitted at the transmitting end undergone different delays, resulting Inter Symbol Interference (ISI) in the received signal. The receiving end usually uses an equalizer to equalize the received signal before performing signal detection so as to reduce ISI and improve the detection performance. However, practical equalizers have limited delays. Thus it is impossible to cancel ISI completely after equalization of the received signal. At this point, if the cross-correlation behavior of two signals is not good, then the false detection rate at the receiving end of the respective UEs for the feedback informations transmitted thereto.
During the HSUPA process, if the signature sequence used by a UE remains unchanged, then it means that the cross-correlations between the respective signature sequences remain unchanged. This a codeword of good cross-correlation performance results in low false detection rate by the UE on the feedback information transmitted thereto, while a codeword of relative poor cross-correlation performance results in relative higher false detection rate by the UE on the feedback information transmitted thereto, leading to an unbalanced situation wherein some of the UEs have good transmission performance for HSUPA, while others have poor transmission performance.